In a known method of controlling a fuel supply quantity for the purpose of properly supplying fuel to an internal combustion engine, a basic supply quantity is determined according to a basic engine operation parameter, such as pressure in an intake pipe, in synchronism with engine speed, and the basic supply quantity so determined is corrected, i.e., increased or decreased, according to an additional engine operation parameter such as engine cooling water temperature or a transitional change of the engine, thereby determining a fuel supply quantity. A fuel supply device such as an injector is then operated for a period of time corresponding to this fuel supply quantity to thereby control the fuel quantity to be supplied to the engine.
In the prior art, when a three-way catalyst is provided in an exhaust system so as to purify an exhaust gas, the three-way catalyst is operated most effectively at an air-fuel ratio of a fuel mixture near a theoretical air-fuel ratio (14.7, for example). Therefore, the air-fuel ratio of the fuel mixture is usually feedback controlled to the theoretical air-fuel ratio, by detecting an exhaust gas component concentration, such as an oxygen concentration in the exhaust gas, as one of the engine operation parameters, by means of an exhaust gas component concentration sensor, and correcting the basic supply quantity according to an output signal from such sensor.
Such an air-fuel ratio feedback control is not always carried out, but may be stopped under specific operational conditions of the engine, such as low cooling water temperature or high engine load, so as to improve the operational condition. Instead, an open-loop control is carried out irrespective of the output signal from the exhaust gas component concentration sensor, so that the air-fuel ratio may be enriched.
Further, in the above-described method, the fuel supply quantity is increased under a high engine load to enrich the air-fuel ratio. It is undesirable to carry out the air-fuel ratio feedback control when increasing the fuel quantity. There is disclosed in U.S. Pat. No. 4,494,512 a control method wherein a high engine load is determined when the fuel supply quantity becomes greater than a predetermined quantity, and the open-loop control is substituted for the air-fuel ratio control.
However, the above-described control method has the drawback that the exhaust quantity of CO (carbon monoxide) . is temporarily increased to reduce the exhaust gas purification rate. To prevent such an increase in the exhaust quantity of CO, it is proposed in Japanese Patent Publication No. 62-126236 that the timing of the shift from the air-fuel ratio feedback control to the open-loop control is delayed for a predetermined time after the fuel supply quantity exceeds the predetermined quantity. However, since the combustion condition of the engine at a low engine temperature is unstable, it is desirable to quickly enrich the air-fuel ratio. For this reason, applicant has determined that the time delay in shifting the feedback control to the openloop control is preferably variable.